#include<stdio.h>
#include<stdlib.h>
#include<math.h>
int main()
{
    double up, down,s;
    int i;
    up = 0;
    down = 1;
    s = 0;
    
    
    for (i = 1; i <= 20; i++) 
    {
       
        if (i % 2 == 0)
        {
            up = -i;
        }
        else
        {
            up = i;
        }
        down = 3*(double)i + 3;
        s += up / down;
       
        

    }
        printf("%.6lf", s);

        return 0;
}
